The present invention relates to a surface-modified composition formed of moldable polymers possessing a lasting hydrophilic nature, to channels for aqueous fluids based on this composition, to a microfluidic system, such as a microfluidic chip, comprising these channels, to its process of manufacture and to a microfluidic chip incorporating said system.
Microfluidic systems, such as microfluidic chips, are composed in a known way of inert biocompatible substrates (i.e., substrates which can be used as cell culture support) on which networks of channels are formed and they are suitable in particular for localizing in a well-ordered way, on these substrates, the deposition of proteins originating from aqueous solutions. These microfluidic systems are of major importance in fields of activity such as the biomedical field, genetics, pharmacology, analytical chemistry, synthetic chemistry or combinatorial chemistry, due to the fact that they make it possible to reduce the measuring or validation times and the volumes of reagents necessary for analyses.
These microfluidic systems are conventionally composed of a flexible polymer, such as a crosslinked rubber composition based on a silicone rubber. Advantageously, a polydimethylsiloxane (PDMS) is chosen as silicone rubber, essentially owing to the fact that it is a widespread, gas-permeable, inert and biocompatible elastomer and that it can be easily molded, making possible, at a low cost, satisfactory reproduction in a micromold of the microstructures intended to form fluidic channels on the micrometer or even nanometer scale. In addition, the transparency to visible light in the absence of autofluorescence phenomenon which characterizes PDMSs allows various methods of microscopic observation of cells within these microstructures.
However, a disadvantage of the PDMSs is that they exhibit a highly hydrophobic nature, which means that the walls of the channels of the microfluidic systems made of PDMS cannot be wetted by aqueous liquids. This is the reason why attempts have been made to develop methods for rendering PDMSs hydrophilic at the surface. It has been known for a long time that plasma surface treatments make it possible to confer a satisfactory hydrophilic state on PDMSs. Nevertheless, it turns out that this hydrophilic state is only short-lived, on account of it coming to an end in the open air after a few hours.
The paper by Papra A. et al. (Microfluidic Networks Made of Poly(dimethylsiloxane), Si and Au Coated with Polyethylene Glycol for Patterning Proteins onto Surfaces, American Chemical Society, Langmuir, 2001, 17, 4090-4095) presents a process designed to confer a longer-lasting hydrophilic nature on a microfluidic system which is, for example, based on a PDMS, this process consisting essentially (cf. page 4091):                in subjecting the surface of the microfluidic system to an oxygen plasma oxidation treatment for ten seconds,        in immersing the system thus treated for ten hours at ambient temperature in a solution of hydrochloric acid in an aqueous medium comprising a poly(ethylene glycol)di(triethoxysilane) with a weight-average molecular weight (Mw) of 3400 g/mol (hereinafter Si-PEG-Si), then        in washing the system obtained, in subjecting it to ultrasound treatments, then in drying it and in cleaning it,        in subjecting the system thus treated to oxidation for ten minutes by a solution based on aqueous hydrogen peroxide solution and on sulfuric acid,        in again washing the system thus oxidized and then in subjecting it to further ultrasound and drying treatments,        in again immersing the system obtained for 18 hours at ambient temperature in said Si-PEG-Si in solution in toluene and in the presence of hydrochloric acid, in order to obtain, by a grafting reaction of the Si-PEG-Si, a coating which adheres to the PDMS, then        in washing the system obtained and then in subjecting it to an ultrasound treatment, in order to remove ungrafted residues.        
This paper indicates that the surface of the PDMS thus modified by grafting retains a hydrophilic nature for approximately three weeks.
It should be noted that the adhesion between the Si-PEG-Si and the PDMS which is obtained by the implementation of these oxidation and grafting stages is achieved by virtue of the formation of covalent bonds of siloxane type.
A major disadvantage of this process lies in its complexity and in the relatively long time which it takes to carry it out, in particular due to the use of solvents, and in the difficulty of commercially obtaining the abovementioned Si-PEG-Si polymer, the only one recommended for the grafting. This results in a relatively high manufacturing cost for this microfluidic system which is highly damaging to the possibilities of applying it industrially. It is necessary to have available processes which are simple, fast and inexpensive to implement in order to retain a low manufacturing cost, which is composed mainly of the production by molding of microfluidic systems with hydrophobic elastomers or thermoplastic polymers.